This invention relates to superconductors in general and more particularly to an improved arrangement for closing a superconducting current path using contacts of a stabilized superconductor material.
Once the magnetic field of a superconducting coil, and particularly that of a high field intensity magnet coil, has been generated, almost no energy need be supplied to the coil from outside in order to maintain the field. The only energy required to maintain the superconducting state of the coil is that associated with the refrigeration devices needed to keep the conductors cooled down. As a result, once the coil has reached this state, a short circuit can be provided which short circuit will preferably be a superconducting short circuit. Once this is done, the current will flow in the circuit so formed such that it is only very slowly attenuated and the current supply needed for the initial excitation of the coil can be interrupted.
A shorting element of this general type for use with high field intensity magnetic coils is disclosed in "Elektrie," vol. 19, No. 4, pages 176 to 182, (1965). To start with, provision for shorting a coil which is connected to a current supply is made between the two terminals of the coil. The shorting connection is arranted so that during the building-up phase of the magnetic field in the coil, it is in a normal conducting state and thus represents a relatively high resistance. As a result, current flows through the low resistance superconducting coil. In well known fashion, the magnet coil to be shorted is placed in a bath cryostat and surrounded by liquid helium as the coolant. The leads of the coil are brought out of the cryostat to the current supply which is at normal temperature. In addition, protruding from the surface of the helium bath is a yoke-like shorting section which is connected to the terminals of the superconducting coil and which has a heater attached to it. With the heater operating, the shorting section stays in the normal conducting state offering high resistance to the current being supplied to the coil. As a result, the current supply from outside, assuming the inductive counter EMF is not too large, flows only through the superconducting coil whose resistance is substantially smaller than the resistance of the shorting path shunted across it. After initial build-up of the current in the corresponding coil, the heater of the shorting element is switched off and/or liquid helium filled in to the cryostat to such a height that the shorting section is immersed in the cooling medium. As a result, the shorting path makes a transition from the normal conducting to the superconducting state and thus, constitutes a superconducting short circuit for the coil. The current can then circulate in the closed superconducting circuit so formed. The leads of the current supply can then be removed in order to avoid any influx of heat through the leads.
As with any arrangement for shorting a superconducting coil, the above described arrangement does not rely on magnetically controlled shorting elements. These are not practical because of the desired large magnetic fields generated by superconducting coils. An element of this nature is described in U.S. Pat. No. 3,339,165. Since in its operating condition the superconducting contact path of the shorting element should permit the highest possible current densities, very high magnetic field densities must then be applied to the shorting element in order to obtain in it what is referred to as quenching, i.e., the transition from the superconducting to the normal conducting state. If the shorting element is arranged in the vicinity of a high field intensity magnet coil which is to be short circuited, these strong additional fields may have a detrimental effect.
The mode of operation described in the above article from "Elektrie" is used not only to save energy but also because the circulating current in the coil is nearly constant. The current is normally chosen so that a certain margin of safety exists from the critical loading point of the coil. Under these conditions, the decrease of the field or the current with respect of time is very small in this short circuited superconducting coil circuit, i.e., the rate of decline is only approximately a few percent a day. As a result of this type of operation, short circuit magnets of this nature can be used for vehicles which are guided and supported using electrodynamic suspension guidance. In such a system, the vehicle is guided at a high speed along an associated stationary track without contact, the lifting and guidance forces being generated by magnetic interactions.
Superconducting high field intensity magnets are generally manufactured using a stabilized type of construction. In a construction of this nature, a portion of stabilizing material such as copper or aluminum is connected in parallel with the superconducting material so that the total current can temporarily be carried by the stabilization material. Through the use of a stabilizing arrangement, the influence of instabilities in the operation of such magnets can be avoided through design measures such as those described for example in Elektrie, Vol. 21, No. 1, pages 1 to 7 l (1967).
In Elektrie, Vol. 23, No. 3, pages 126-128 (1969), it was noted that a stabilized superconductor which is used as a shorting element of a stabilized superconducting coil can be used only if a section of the conductor is freed of its stabilizing material by a pickling or etching operation, for example. A superconducting section of this nature can furthermore be heated in order to produce quenching therein. Through these two measures, a sufficiently high resistance between the terminal points of the coil winding during the excitation of the high field intensity magnet is obtained.
During the excitation process of the magnet, relatively high helium evaporation losses can occur in such a heated normal conducting section of the shorting element. Furthermore, the stabilizing properties for the short circuit operation of the magnet coil are lost at the point of short circuit due to the pickling or etching operation. Should an accident occur, this can lead to excessive heating of this particular point to a degree such that the conductor melts.
A type of switch which can be mechanically actuated and which contains two contacts is described in German Auslegeschrift 1,615,591. The contact elements which are referred to as a switch reed and a switch block each consist of two strip-shaped stabilized superconductors. The switch has a number of essential mechanical parts as follows: the switch reed; the switch block which receives and seizes the switch reed; a plunger and a member by means of which the switch reed can be set to the switch block; and a shaft by which cams can be operated to bring the conductors of the switch block and the switch reed into contact with each other under pressure using pressure plates. The conductors of the switch reed are rotated approximately 90.degree. with respect to the common switch base plate in the region of the switch block so that the switch reed which is perpendicularly movable at the plunger with respect to the base plate can be moved into the switch block with its conductors perpendicular to the base plate. Similarly with this arrangement, retraction is possible moving in the opposite direction. Thus, for closing the switch, the plunger is pressed down causing the two conductors of the switch reed to be brought into one position relative to the conductors of the switch block. Thereupon, the shaft is rotated in the process of which the cams are operated which then, by means of pressure plates, bring the exposed conductor surfaces into contact with each other under pressure and keep them in this position. It can be seen that the switch described in this reference is a complicated mechanism.
Since large mechanical switching resistance must be overcome where stabilized superconductor ribbons for large currents are used due to the bending of the conductors for the switching process, a sturdy design of the actuating rods of the shaft and the plunger is necessary. These rods, whose one end is at normal temperature, can cause an undesirable introduction of heat into the helium bath used as a cooling medium and as a result, additional helium losses can occur.
Thus, it can be seen that there is a need for an improved element for closing a superconducting current path such as for short circuiting a superconducting magnet, which element is an improvement over the prior art mechanical switch and heated type shorting arrangement of the prior art. Such a device for short circuiting a path in this nature should have a simple mechanism and result in a minimum of helium losses.